The gastrointestinal tract as an endocrine/neuroendocrine/paracrine organ: organization, chemical messengers and physiological targets

The fish gut holds special status among internal organs since its lumen is continuous with the external medium, giving the gut direct contact in a technical sense with the outside world. Consequently, the multifunctionality of the gut extends to include luminal sensing by epithelial cells and signaling to physiological targets both near (paracrine/autocrine/luminocrine signaling) and far (traditional endocrine signaling by way of the blood vascular system). The chemical agents for this signaling are produced by cells of the gastroenteropancreatic series. Additionally, neuropeptides and neurotransmitters from axon terminals of the peripheral nervous system contribute to regulatory signaling at the gut. Combined, the humoral factors of these systems operate in parallel locally to direct digestive processing (secretion of digestive enzymes, and motility), to regulate nutrient and ion/water absorption, and to influence the secretion of other hormones, and more distantly at the brain to modulate feeding and drinking, and at various tissues and organs to regulate metabolism, growth and other essential physiological processes. This chapter summarizes the current state of knowledge regarding the gut as a hormone-secreting organ in fishes, and surveys key target actions in the gut and elsewhere of gastroenteropancreatic series chemical messengers.